A non-volatile memory device is capable of retaining stored information even when power to the memory device is turned off. Traditionally, non-volatile memory devices occupied large amounts of space and consumed large quantities of power. As a result, non-volatile memory devices have been widely used in systems where limited power drain is tolerable and battery-life is not an issue. However, as systems requiring non-volatile memories have continued to shrink in size, improvements in non-volatile memory devices have been sought in order to make these devices more suitable for use in portable electronics or as substitutes for frequently-accessed volatile memory devices. Desired improvements include decreasing the size and power consumption of these memories and improving the memory access devices.
Improved non-volatile memory devices under research include resistive memory cells where resistance states can be programmably changed. Resistive memory cells store data by structurally or chemically changing a physical property of the memory cells in response to applied programming voltages, which in turn changes cell resistance. Examples of variable resistance memory devices being investigated include memories using variable resistance polymers, perovskite materials, doped amorphous silicon, phase-changing glasses, and doped chalcogenide glass, among others. Phase change memory (“PCM”) cells have varying resistances as a result of changes in the phase of the cell material. Spin-tunneling random access memory (“STRAM”) cells have varying resistances as a result of changes in current induced magnetization of the cell material.
For many resistive memory cells, changing the cell resistance is accomplished by passing an electrical current of sufficient strength through the resistive memory cell. For phase change memory cells and spin-tunneling memory cells, for example, programming and reset currents of 50 to 100 μA are not uncommon. However, these high currents result in extremely high current densities as the size of the memory cells continues to shrink. For example, for a 20×20 nm2 memory cell, the resulting current density is of the order of 1×107 A/cm2 or greater. For such high current densities, improved memory access devices are desired to provide high currents and low “off” state leakage.
Improved access devices such as those desired for use with resistive memory cells could also be used to provide high currents to any type of memory or semiconductor circuit that requires a high current.